Thermo_Part3_Slidesonly_18.pptx

# Thermo_Part3_Slidesonly_18.pptx - BC3232y Thermodynamics...

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1 BC3232y: Thermodynamics Review Part 3 James Joule

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2 General equation Reaction α A + β B γ C + δ D Δ r H m ° = γ Δ f H m °(C) + δ Δ f H m °(D) – α Δ f H m °(A) – β Δ f H m °(B) units : α, β, γ, and δ are dimensionless: they specify (moles substance)/(moles reaction) Δ r H m ° and Δ f H m ° values have same units: kJ/mole the mole in the denominator is moles of reaction as written (this is why the reaction must appear)!
3 Heats (enthalpies) of combustion Enthalpies of formation difficult to measure directly. For hydrocarbons (compounds with C, H, and O), combustion reactions generally occur cleanly. Combustion combines the specified molecule with O 2 (g) to form H 2 O and CO 2 Enthalpies of combustion often measured directly; tabulated enthalpies of formation determined from them! Enthalpies of combustion (in tables) can also be used directly to determine reaction enthalpies.

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4 Heats (enthalpies) of combustion Definition: The enthalpy of combustion , Δ comb or Δ c , is Δ r H° for the reaction in which one mole of the specified substance combines with O 2 (g) , forming H 2 O(ℓ) and CO 2 (g) . Note that by convention, liquid water is the product, unless stated otherwise. Why liquid? It is the stable form at 25°C.
5 Heats (enthalpies) of combustion Example: combustion of ethanol C 2 H 5 OH(l) + 3 O 2 (g) 2 CO 2 (g) + 3 H 2 O(l) Burn ethanol in a calorimeter, measure Δ c H°: Δ c H°{EtOH(l)} = -1366.82 kJ/mole

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